In recent years there has emerged a dispute whether Hamotzi or Mezonot should be recited on one slice of pizza. We will present the background to this issue in the Gemara, Rishonim, and Acharonim. Then we will explain the two sides of the contemporary debate on this issue.

Gemara and Rishonim
The Gemara (Berachot 42a) records the rule that one should recite Mezonot on “Pat Habaah BeKisnin” as long as he does not “establish a meal” (Kovei’ah Seudah) on it. If, however, he does establish a meal on Pat Habaah BeKisnin then he must recite Hamotzi. The Rishonim debate two major points about this Gemara. First, what exactly is Pat Habaah BeKisnin? Second, how do we determine that one has “established a meal” on Pat Habaah BeKisnin?
We will first examine the issue of when is one considered to have established a meal on Pat Habaah BeKisnin. The Rishonim argue if establishing a meal is determined by every individual according to his particular standard or by what most people consider constituting a meal. The Rosh (Berachot 6:30) cites the Raavad who believes that each individual determines what is considered to be a meal by his own individual standard. The Rashba (commenting to Berachot 42a), however, disagrees and believes that it is determined by what most people consider a meal. The Rosh agrees with the Rashba. The Shulchan Aruch (Orach Chaim 168:6) rules in accordance with the opinion of the Rashba and the Rosh. Thus one must recite Hamotzi if he has eaten an amount of Pat Habaah BeKisnin that most people consider a meal, even though by his personal standards he is merely consuming a snack (also see the varying opinions cited in the Mishna Berura 168:24).
It should be noted that the Biur Halacha (168b s.v. Af Al Pi) asserts that the “objective standard” varies from group to group. He specifically mentions a distinction between young and old people. Thus, if an older individual plans to eat an amount of Pat Habaah BeKisnin that most older people consider to constitute a meal, he must recite Hamotzi even though a younger person would view that amount of food as merely a snack. Rav Daniel Wolf of Yeshivat Har Etzion (Yeshivat Har Etzion’s Daf Kesher volume 6) suggests that a distinction can be made between men and women regarding this matter, as men generally eat more than women. I have often wondered whether adolescent boys should be considered as a distinct group regarding this matter as well, since they tend to eat more than most other people. Personally speaking, I considered three slices of pizza to constitute a meal when I was a teenager, and as an adult I consider two slices of pizza to constitute a meal.
It should be noted that, according to many opinions, one need not eat an entire meal of Pat Habaah BeKisnin in order to establish a meal. One must recite Hamotzi when he plans to eat some Pat Habaah BeKisnin along with enough food to establish a meal (Mishna Brura 168:24 and Teshuvot Igrot Moshe O.C. 1:56 and 3:32; see, however, the Aruch Hashulchan Orach Chaim 168:17 who disagrees).
Interestingly, Rav Yaakov Kaminetzsky (cited in The Halachos of Brachos p. 261 note 6.1 and Emet LeYaakov Orach Chaim 168, note 196) argues that this rule does not apply if the Pat Habaah BeKisnin is consumed only as dessert. Thus, for example, if one eats potato pie or crackers along with the main course, he must wash and recite Hamotzi and Birkat Hamazon but if he eats a full meal and then has cake for dessert he is not required to recite Hamotzi and Birkat Hamazon. Rav Shlomo Zalman Auerbach (cited in Shmirat Shabbat Kehilchata 54 note 132) similarly suggests that only when the Pat Habaah BeKisnin is consumed together with meat or fish (i.e. the main course) does the meat or fish combine with the Pat Habaah BeKisnin to establish a meal. Rav Moshe Feinstein, though, rules that even cake eaten for dessert combines with the rest of the meal to be Kovei’ah Seudah and thereby require one to recite Hamotzi and Birkat Hamazon. Rav Mordechai Willig (Am Mordechai page 101) discusses this issue at some length and rules in accordance with the views of Rav Yaakov Kaminetzky and Rav Shlomo Zalman Auerbach.

What is Pat Haba’ah BeKisnin?
Next we will review the three opinions in the Rishonim regarding the definition of Pat Habaah BeKisnin. Rabbeinu Chananel and the Aruch define it as “bread that is baked with a pocket full of honey, sugar, nuts, or spices.” This appears to be similar to what we call pie. The Rambam (Hilchot Berachot 3:9) defines it as “dough which has honey, oil, milk, or spices mixed in it.” This appears to be similar to what we call cake. Rama (O.C.168:7) explains that it is called cake only when a considerable amount of spices or honey is added to the dough. Mishna Berura (168:33) explains that it is considered a “considerable amount” only when the spices or honey is a dominant taste. Only then is it defined as cake and not as bread.
Parenthetically, it should be noted that this is one of the reasons that Rav Aharon Lichtenstein, Rav Hershel Schachter, Rav Mordechai Willig and virtually every Rav that this author is acquainted with believe that Hamotzi should be recited over “Mezonot bread.” Even though apple juice is added to the dough in the baking of “Mezonot bread”, its Bracha is Hamotzi because the taste of apple juice is hardly noticeable. Moreover, people treat "Mezonot bread" exactly as bread and thus cannot qualify as Pat Habbah BeKisnin that by definition is a snack type of food. For a full discussion of the fallacy of “Mezonot bread”, see Rav Binyamin Forst’s The Laws of Berachos pages 253-256.
The third opinion regarding the definition of Pat Habaah BeKisnin is the view of Rav Hai Gaon, who believes that it is crackers. Interestingly, it is for this reason that Sepharadim recite Mezonot when they eat Matzah (as a snack) other than during Pesach (see Rav Ovadia Yosef Teshuvot Yechave Da’at 3:12). See, however, Rav Eliezer Waldenberg (Teshuvot 11:19) for a full defense of the Ashkenazic practice to recite Hamotzi on Matzah (even if eaten as a snack) year round.
Interestingly, the Shulchan Aruch (168:7) rules in accordance with all three views. Indeed, many Acharonim query as to why the Shulchan Aruch decided to do so. One suggestion is that the Rishonim do not disagree but are simply offering different examples of what is Pat Habaah BeKisnin. Thus all agree that pie, cake and crackers are considered Pat Habaah BeKisnin (see Biur Halacha s.v.Vehalacha and Rav Mordechai Willig’s Am Mordechai page 100).
The Shulchan Aruch (168:17), however, writes that dough baked when filled with meat, fish, or cheese requires the bracha of Hamotzi. The Mishna Berura (168:94) explains that Hamotzi is required even if he did not “establish a meal“ on these foods. This is because these are “meal type foods” and not snacks, in contradistinction to pie, cake, and crackers. This rule is not absolute as the Mishna Berura rules that “franks in a blanket” are snacks and Mezonot is recited on it (unless one establishes a meal on it).
An analysis of the Pat Habaah BeKisnin rule might help us understand this ruling. Rav Meir Lichtenstein told me that his grandfather Rav Yosef Dov Soloveitchik believes that fundamentally the Bracha of Hamotzi is not a Bracha on bread. Rather, it is a Bracha recited upon a meal (Seudah). According to this insight, one recites Mezonot on Pat Habaaah BeKisnin even if it is formally defined as “bread”, since one is consuming it as a snack and not as a meal. However, if one eats Pat Habaah BeKisnin as part of a meal then it follows that one should recite Hamotzi on the Pat Habaah BeKisnin. Similarly, the Bracha of Mezonot is appropriate only for cake or crackers since they are “snack foods” and not for meat or cheese pies since they are “meal foods.”

Contemporary Rabbinical Authorities
The debate whether the bracha for one slice of pizza is Mezonot or Hamotzi hinges on the question if pizza is viewed as “meal food” or “snack food.” Many people recite Mezonot on one slice of pizza based on Rav Moshe Feinstein’s reported ruling that pizza is Pat Habaah BeKisnin. Rav Moshe is reported to have asserted that pizza is a snack type food and that one does not establish a meal when he eats only one slice of pizza. Other Poskim disagree. Rav Mordechai Willig (Am Mordechai page 99) rules that Hamotzi should be recited even on one slice of pizza because most often pizza is consumed on the context of a meal and not as a snack. This argument might hinge on what is meant as a “meal.” Rav Moshe might respond that the Halacha refers to a full meal such as dinner and not lunch, which in America is regarded as a light meal (see Rav Forst, The Laws of Berachos page 249 footnote 77 who advances a similar argument). Rav Ovadia Yosef (cited in Yalkut Yosef , Kitzur Shulchan Aruch, page 223 in the 5760 edition) and Rav Yisroel Belsky (Mesora 1:40) also rule that one should recite Hamotzi even on one slice of pizza. Rav Zalman Nechemia Goldberg and Rav Hershel Schachter also told this author that Hamotzi is the appropriate Bracha even for one slice of pizza. Rav Willig, however, notes that pretzels are considered Pat Habaah BeKisnin because it is commonly eaten as a snack.
It appears to me that the reason why common practice seems to accord with Rav Moshe’s view (aside from the convenience factor) is that when kosher pizza was first introduced in the United States in the 1970’s, Rav Moshe Feinstein was the preeminent Halachic authority in America at that time. It appears that the authorities who disagree with Rav Moshe issued their rulings only after Rav Moshe’s ruling became entrenched among Am Yisrael. One should consult his Rav for a ruling regarding this matter.